In integrated circuit technology, it is often desirable to have two or more independent subcircuits for performing individual functions. However, it is desirable to limit the number of pins used on an integrated circuit device.
In prior art devices, multiplexers have been used to send a signal from one pin to one of two or more circuits. For example, in a memory chip, the same data pins are used for both read and write functions. However, this circuitry requires that a control signal be input to the integrated circuit, thereby necessitating an additional pin (e.g., the "R/W pin).
In some instances, it may be desirable to have a subcircuit which is not used during normal operation of the integrated circuit. For example, it may be desirable to provide a circuit which is capable of testing the operating characteristics of the overall integrated circuit. This subcircuit would only be used while the integrated circuit was not in an operating mode. Hence, any pin dedicated to the testing subcircuit would be overhead during the normal operation of the circuit. Therefore, a need has arisen in the industry for a method and apparatus for selectively enabling one of two or more subcircuits without the need for additional packages pins.